The present invention relates to the diagnostic arts. It finds particular application in conjunction with determining iron concentration in the liver of human patients and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in determining the presence of ferrous or other ions which affect magnetic susceptibility in other human tissues, as well as in non-human subjects.
Previously, liver iron levels have been quantized in MRI scanners by correlating susceptibility altered T.sub.2 decay with iron levels in the liver. For a liver with normal iron levels, i.e. about 10 .mu.g/ml, the T.sub.2.sup.. relaxation time is about 40-50 msec. The T.sub.2.sup.. relaxation time becomes shorter with increasing iron levels. In extreme cases, where the iron concentration is on the order of 150 .mu.g/ml, the T.sub.2.sup.. relaxation time becomes less than 2 msec. Commercial MRI scanners typically have a spin echo minimum time of about 6 msec. This reduces the signal strength by about 95% and introduces an error or variance of about 50% into the measurement of the T.sub.2.sup.. relaxation time.
Outside of the MRI environment, liver iron levels have been measured using SQUID technology. A water bath is disposed between the patient's liver and a SQUID pick-up coil to provide a zero reference. The SQUID coil monitors the actual magnetic field from the liver itself. The relationship between the strength of the liver generated magnetic field and the liver iron concentration have been determined experimentally. When examining a patient, this relationship is consulted to convert the monitored magnetic field strength into a liver iron concentration value.
The present invention contemplates a new and improved technique for measuring liver iron concentration, particularly higher concentrations, in an MRI environment.